Graft-versus-host (GVH) reactons were measured using a quantitative spleen weight assay. Donors were Balb/cN mice raised in pathogen free conditions for 6 weeks, then transferred to a conventional environment. GVH activity in C57BL/6N x Balb/cN newborns measured 6-8 days after transfer showed that the activity of cells from thymus, spleen and lymph nodes were higher than seen previously with genetically similar mice of the same age; and that activity rose to a peak about 23 days after the mice were transferred, then decreased to levels higher than those seen with cells from 7 week old donors. Other factors than genetic difference appear to influence the GVH activity of lymphocytes. BIBLIOGRAPHIC REFERENCE: Morse, H.C. III, Pasanen, V.J., and Asofsky, R.: Differential effects of alloimmunization on T cells mediating graft-verus-host splenomegaly or the allogeneic effect in F1 mice. J. Immunol. 118: 574-577. 1977.